1. Field of the Invention
This invention relates to a sensor for detection of the extent of strip edge of mostly metallic electro-conductive materials inserted between the electrodes comprising the detector.
2. Description of the Prior Art
Various methods have been devised and utilized to sense the edge position of a strip of electro-conductive material, including such means as photoelectric, image-processing, or electromagnetic methods. These methods have been employed accordingly for specific purposes with their advantages and short-comings taken into consideration.
In addition to these methods, a method in practice is the means for detection of the change in capacitance or its mediated variables. In principle, the method in use of capacitance further has two variations. The first method utilizes the change in capacitance itself, as shown in FIG. 1. The high-frequency voltage source 101 is applied between the electro-conductive strip 100 and the electrode 102, so that the insertion extent X of the strip 100 into the electrode 102 gives rise to the change in capacitance which is detected as the change in resonant frequency associated with the tuning coil 104. The change in the resonant current thereof is amplified by the amplifier 103 to evaluate the insertion extent X.
In the second method the evaluation of the extent is performed through a signal mediated by means of capacitance such as the change in high-frequency voltage with insertion of an electro-conductive material into a capacitor. FIG. 2 illustrates an exemplary embodiment of this method. The high-frequency voltage source 101 is applied on the transmitting electrode 102a, so that the current change responding to the portion X of the electro-conductive strip 100 inserted between the transmitting electrode 102a and the receiving electrode 102b is amplified by the amplifier 103 to detect the insertion extent X.
The device of the second method has an input terminal and an output terminal, between which a function is defined so that the output of the function varies with the inserted position of the electro-conductive strip. The function is simply expressed as
Y=Kxc2x7Xxe2x80x83xe2x80x83(1)
where Y is the output such as voltage and K is a coefficient X denotes the insertion extent of the strip between the electrodes.
The present invention described in the followings is associated with the type of the second method, which is hereafter called xe2x80x9celectrostatic three-terminal model.xe2x80x9d The exciting signal source commonly used for the system of electrostatic three-terminal model is the high-frequency voltage, which the present invention also utilizes. This selection, however, arises simply from a practical feasibility, and the present invention may be theoretically realized employing an alternating voltage with any frequency above 0.
As Equation (1) above indicates, the function is desired to be sensitive only to X, the insertion extent of the strip between the electrodes. In the actual operation, however, several factors cause some fluctuation in the coefficient. The following Equation (2) is obtained by adjusting Equation (1) closer to the reality:
Y=(1+xcex1)xc2x7Kxc2x7Xxe2x80x83xe2x80x83(2)
where xcex1 represents the fluctuation of the coefficient due to, for example, complex change in space dielectric constant owing to temperature, humidity or pressure in atmosphere, variation in the distance between the transmitting and receiving electrodes, or electric reflection and/or leakage in the surroundings. Notwithstanding these effects, Equation (2) has yet been used assuming that the variation in xcex1 is negligible or very small. This situation naturally imposes a serious restriction on practical use, limiting the applicable range and facility.
It is accordingly an object of this invention to overcome the difficulties described above and to provide a sensing device for detection of the insertion extent of electro-conductive strip between the electrodes isolated from the surrounding physical conditions.
The essential concept in this invention is based on the assumption that the fluctuation of a in Equation (2), that is, interference by the various factors, spreads uniformly in the physical space occupied With the detector. For explanation of the solution to the difficulties, the basic principle common to the electrostatic three-terminal model is described at first with reference to FIG. 3. An electro-conductive strip 1 is inserted by x between the transmitting electrode 2 and the receiving electrode 3 disposed facing to the electrode 2. The high-frequency voltage source 4 supplies the alternating voltage to the transmitting electrode 2 to form the electric flux between the electrodes 2 and 3. The first approximation only is considered here, so that the fringe effect of capacitance and the curvature of electric flux are neglected for simplicity of explanation.
Under this condition, the electric flux distributes uniformly on the whole electrodes with uniform strength of electric field. As shown in FIG. 3, the electric flux concentrates into the current which flows in the resistor 5. If the impedance in the part yielding capacitance is sufficiently large compared with the resistance of resistor 5, the voltage generated in the resistor 5 is proportional to the total number of electric flux concentrating on the receiving electrode 3. FIG. 3 illustrates the way in which the electric flux diverges into two groups, which concentrate respectively into the resistor 5 and the electro-conductive material 1 to be measured. As the density of electric flux is dependent upon the strength of electric field, the density of group a of the flux flowing in the receiving electrode 3 is different from that of group b of the flux flowing in the electro-conductive material 1. This difference, however, causes no harm for the measurement since the measurement in this system concerns solely the group a of the flux. In fact, the current flowing through the resistor 5 is dependent upon the insertion extent x of the electro-conductive material 1. The relationship between them is linear for the first approximation, and expressed as
IL=k(1xe2x88x92x)
where IL is the current flowing through the resistor 5, and k is a proportional coefficient. The length of the two electrodes (distance in insertion direction for the electro-conductive material 1) is normalized to 1. Incidentally, the object of the present invention is to correct the measurement errors resulting from every disturbing factors affecting the density of the group a of the flux.
The case is concerned so far with only one frequency for the applied voltage, or the electric flux generated thereby. The case for multiple frequency components in the electric flux is now considered. Specifically, when the frequency has two components f1 and f2, the numbers of the electric fluxes with these frequencies are assumed to be linearly distributed as follows:
EMf1x=xxe2x80x83xe2x80x83(3)
EMf2x=1xe2x88x92xxe2x80x83xe2x80x83(4)
where EMf1x and EMf2x, represent the numbers of the electric fluxes at x with the frequencies f1 and f2, respectively. x denotes the insertion extent of the strip between the electrodes and the length of the two electrodes (distance in insertion direction for the electro-conductive material 1) is normalized to 1.
FIG. 4 shows the distribution of the number of the electric flux by the arrow length, where the full and broken lines indicate the numbers of the electric fluxes obtained from Equation (3) for frequency f, and Equation (4) for frequency f2, respectively. As seen in FIG. 3, the current running through the resistor 5 is proportional to the integral of the electric flux reaching the receiving electrode without interruption by the electro-conductive material 1. With the range of x set to be 0 less than x less than 1, integration from x to 1 leads to the following expression for the current:
ILf1x=∫EMf1xdx=xc2xdk(1xe2x88x92x2)=xc2xdk(1xe2x88x92x)(1+x)
ILf2x=∫EMf2xdx=xc2xdk(1xe2x88x92x)2=xc2xdk(1xe2x88x92x)(1xe2x88x92x)
where ILf1x and ILf2x indicate the currents flowing through the resistor 5 generated with the electric fluxes respectively corresponding to the frequencies f1 and f2. The ratio of ILf2x to ILf1x hence is obtained as                                                         I              L                        ⁢                          f                              2                ⁢                x                                                                        I              L                        ⁢                          f                              1                ⁢                x                                                    =                              1            -            x                                1            +            x                                              (        5        )            
the value of which varies continuously from 1 to the limiting value 0.
FIG. 5 displays the relationship of this ratio with the insertion extent x on a graph with the current ratio ILf2x/ILf1x on the ordinate and the normalized insertion extent x on the abscissa. The relationship is obtained theoretically, so that some difficulty might arise around the limit with x=1. From the practical point of view, however, this can be overcome by restricting the maximum value of x, for example, less than 0.8. This current ratio can uniquely correspond to the value of x, leading to the measuring system to determine the insertion extent x of the electro-conductive material into the electrodes. It should be noted that the term k is eliminated by taking the ratio of the currents of two frequency components, implying that for the first approximation the various factors affecting k such as energization voltage, space dielectric constant, the distance between the transmitting and receiving electrodes, and the size of the electrodes influencing the total number of electric flux have no effects on measurement Moreover, applications implicated by the nonlinear relationship in FIG. 5 might become feasible by means of linearization such as nonlinear amplifiers or look-up tables.
Another operational treatment is further performed as follows. The sum ILA is obtained by addition of ILf1x and ILf2x:
ILA=xc2xdk(1xe2x88x92x){(1+x)+(1xe2x88x92x)}=k(1xe2x88x92x)
The difference ILD between ILf1x and ILf2x also is derived as
ILD=ILf1xxe2x88x92ILf2x=xc2xdk(1xe2x88x92x){(1+x)xe2x88x92(1xe2x88x92x)}=k(1xe2x88x92x)x
A variable gain amplifier now is introduced to the receiver system. The gain v for the amplifier is given by
ILA=vk(1xe2x88x92x)
whose value may always be held at any fixed value C by appropriate adjustment of v. With k(1xe2x88x92x) C, hence, the current difference ILD can be written as
ILD=Cx
                    x        =                              I            LD                    C                                    (        6        )            
The insertion extent x into the electrodes thus can be directly related proportionally to the output current difference ILD. This relationship again is not dependent upon k. For this case as well, the maximum value of x is desired to be lower than some value allowable in the actual design, for example, less than 0.8 because, with the limit of x=1, v diverges in order to hold ILA at C.
As explained above, in the present invention the distribution of the number of the electric flux generated between the transmitting and receiving electrodes is formed so as to increase or decrease monotonously from the end of the electrode as indicated in FIG. 4. The current produced on the receiving electrode in response to insertion by x of the electro-conductive material from the end of the electrode is operationally treated to provide ever-stable measurement of x, independent of various kinds of environmental coefficients.